A wavelength division multiplex (WDM) system has transmitted an optical signal in which an optical path signal between the WDM system and, for example, an optical transport network (OTN) is subjected to optical wavelength division multiplexing. However, in recent years, a flexible operation of a transmission path has been desired in association with the complicated network configuration such as a ring network and a mesh network.
When a transmission path is changed due to disconnection of the transmission path on a network and the like, the WDM system also increases or decreases the number of wavelengths of signals passing through each optical transmitter on the transmission path. In an optical amplifier inside each optical transmitter, an auto gain control (AGC) system is used for promptly coping with a variation in optical power of an optical signal corresponding to a variation in the number of signal wavelengths. The optical amplifier in the AGC system compares input power of an optical signal in an amplifier input stage with output power of the optical signal in an amplifier output stage, and controls an amplification factor to be a certain amplification factor corresponding to a power difference resulting from the comparison.
In the WDM system, a technique has been developed for transmitting optical signals in which, in addition to optical path signals, optical packet signals generated in a burst manner between the WDM system and a local area network (LAN) such as an Ethernet (registered trademark) network are subjected to optical wavelength division multiplexing. In the WDM system, because optical packet signals are generated at user's use timing, in other words, at random, therefore, optical packet signals exist in a burst manner in an optical signal. In addition, an optical packet signal has a random signal length and signal interval. In an optical transmitter in the WDM system, in this manner, input power of an optical amplifier greatly varies depending on the presence of an optical packet signal in an optical signal. Related technologies are described in Japanese Laid-open Patent Publication No. 2011-243765, for example.
It is assumed that input power is stably input to the AGC-system optical amplifier for a certain period of time. However, in the WDM system transmitting an optical signal in which an optical path signal and an optical packet signal are subjected to optical wavelength division multiplexing, optical packet signals is mixed in an optical signal in a burst manner. This results in that input power of the optical amplifier greatly varies depending on the presence of an optical packet signal in an optical signal.
The AGC-system optical amplifier has difficulty in ensuring a stable amplification factor because input power varies depending on the presence of an optical packet signal in an optical signal and a power difference thereof varies. When an optical packet signal exists in an optical signal in an amplifier input stage and no optical packet signal exists in the optical signal in an amplifier output stage at an identical monitoring timing, a power difference between the input power in the input stage and the output power in the output stage varies and the optical amplifier has difficulty in ensuring a stable amplification factor.